Optoelectrical measuring system and apparatus

ABSTRACT

The invention relates to measuring the surface roughness of a sample. A beam of light is directed onto the surface and the scattered light distribution is measured using a detector array. Either the average deviation or the second moment of the scattered light distribution is then determined, which are measures of the surface roughness.

This is a continuation of U.S. Ser. No.: 301,969 Filed: 9/14/81

This invention relates to an optoelectronic measuring method fordetermining the surface quality of dispersively reflecting surfaces, inparticular on metallic work pieces, in which the surface to be examinedis illuminated by a generally parallel beam of rays from a light sourceand the intensity distribution of the reflected radiation is measuredand evaluated electronically by photoelectric detectors, and toapparatus for implementing the measuring method.

Measuring methods and apparatus of this type are extremely significantfor ensuring the quality of mechanically manufactured work pieces,because the observance of a specific surface quality, for example, onfitting surfaces, close surfaces, friction surfaces and lacqueredsurfaces is essential for the function of the respective component part.

Various methods and devices are known for measuring the surface qualityor surface roughness. Stylus instruments are used very frequently formeasuring roughness. They have a small probe in which a diamond needlemechanically traces the surface. The measured values are recorded and/orregistered in an amplified manner. These devices have attained a hightechnical level. A disadvantage is the linear tracing of the surface andthe time-consuming measuring procedure which cannot be carried out freeof contact and may only be automated with difficulty.

In the book `Technische Oberflachenkunde`, Verlag J. Springer, Berlin1936, G. Schmaltz describes a measuring method on pages 98 to 99, inwhich the backscatter indicatrix which is detected using movablephotodetectors is evaluated photometrically. Either a factor is used asa measurement for the surface quality, which factor describes theapproximation of the scattered light curve to a Gaussian distribution,or the half-power angle is used, at which the radiation intensity isreduced to half the intensity which is radiated at the specularreflection angle. This method starts out in a restrictive manner from aGaussian distribution of the scattered light curve, or only two measuredvalues for characterising the surface quality are used from the completescatter curve. Apart from the fact that the pre-requisite of a Gaussiandistribution of the scattered light curve is not provided for mostsurface structures, the evaluation which is based on only two measuringpoints leads to inaccurate measuring results.

A measuring apparatus is known from German Auslegeschrift No. 2,260,090which uses the half-power width of the scattered light distributionabout the specular reflection angle of between 60° and 85° angle ofincidence as a measurement for the roughness. This measurement is formedfrom a total of three measuring points of the scattered lightdistribution, that is, the measured value at the peak of the curve andthe respective half-values. In this case, there is a disadvantage inthat the half-width value is not clearly defined on flat ormultiple-peak distribution curves, as occurs in technical roughnessesand it is an unreliable measurement in the statistical sense. Chancefluctuations in the measured values, in particular in the maximum valuewhich represents the reference value have a direct effect on themeasuring result. The flat incidence of light necessary for measuringrenders the measuring arrangement sensitive to changes in spacing anddue to its large dimensions, the measuring apparatus may only be usedwhen there is enough space and time available for the complicatedhandling.

Furthermore, a method is known from the publication by F. Piwonka undTh. Gast in "Technisches Messen" (1979) 12, P. 451-458 in which rotatingphotoreceivers are used to record the backscatter indicatrix and thedepth of roughness is calculated therefrom. This method may only beapplied to surfaces which are machined periodically, for example on arotary machine, because the groove width of the rough grooved surfacemust be known. The measuring range of this arrangement only covers thecoarse range of surface roughness.

Another method for measuring roughness is known from German Patent No.2,241,617 which operates with a laser. In this method, the flatmeasuring sample is rotated mechanically and the angular-dependent lightreflected back in the direction of incidence is measured. Theprobability distribution of the partial derivatives of the roughnessprofile is calculated from the light distribution. The physical boundaryconditions to be observed in this method and the necessary relativemovement of the measuring apparatus and of the object to be measuredconsiderably restrict the area of use.

Furthermore, methods for measuring roughness are known from theliterature which evaluate the "speckle" pattern in the scatter field ofthe reflected light. For this purpose, an illumination apparatus isnecessary which meets certain coherency conditions, for example, alaser.

An object of the present invention is to provide a measuring method andmeasuring apparatus which allow a surface quality to be determined moreaccurately using simple and manageable devices.

Using the measuring apparatus, measurements on flat, convex and concavemeasuring surfaces and at difficult measuring points such as on bevels,in grooves and bores may be carried out free of contact or with gentlecontact in a rapid measuring sequence. The measuring apparatus may beoperated manually and it may also be easily incorporated into automaticapparatus, in which case it is unnecessary to work with coherent light.

Based on the initially-mentioned measuring method, this object isachieved according to the present invention in that a beam of rays isdirected generally perpendicularly onto a surface to be examined and thespatially distributed reflected radiation coming out from theilluminated surface section (measuring spot) is detected byphotoelectric detectors at a solid angle which is adapted to the spreadto be expected of the beam as a result of the scatter reflection andthat integral characteristic values are formed from all of the measuredvalues established by the detectors as a measurement for the surfacequality. Further developments of the measuring method according to thepresent invention and of the apparatus for implementing the method arespecified in the subclaims.

The basic concept of the present invention essentially lies in detectingthe scattered radiation reflected by the measuring spot using ameasuring tube at a solid angle which is adapted to the scatter and thusto the expected quality range of the surface to be examined and informing integral characteristic values S_(x) from the intensity valuesmeasured with detectors in the manner of power moments known frommechanics and statistics, which characteristic values S_(x) are used asa measurement for the surface quality.

The following equations are used to form the characteristic values:##EQU1## The reference in the equations represent the following: w_(i)the angle of the scattered radiation detected by the respective detectori

n the number of detectors used for evaluation

w the average from values p_(i) and w_(i)

p_(i) measurement signal D_(i) standardized according to the equation(c)

g_(i) correction factor for the measurement signal D_(i).

In the method according to the present invention, values p_(i)standardized according to the equation (c) are obtained from themeasured signals of the detectors n which are preferably positioned in arow, taking into account the correction factors g_(i). In contrast tothe known methods and arrangements described above, the measurementsignals of all the detectors recording the reflected radiation enterinto the evaluation. Due to the formation of the quotient p_(i), onlythe scatter characteristics of the surface structure are evaluated andthe material-conditioned spectral reflection coefficients remain withoutinfluence on the measuring result. An average w is calculated from thevalues p_(i) and the values w_(i). The characteristic values S₁ or S₂are formed last of all. The quadratic scatter characteristic value S₂ isthe statistically reliable value. On the other hand, it is easier tocalculate the characteristic value S₁.

The correction factors g_(i) are to balance manufacturing tolerances inthe electrical and optical characteristic values of the constructionelements used and to correct the changes in the scattered lightdistribution determined in a geometrically-optical manner by opticalconstruction elements.

For this purpose, the factors lying between 0 and 1 are determined in ameasuring procedure with a comparative surface of known scatter.

Moreover, it is also possible by approximation of these factors toinfluence the characteristic line path of S₁ or S₂ over a greater rangeof the surface quality by a different weighting of the scattered lightcurve, for example in order to carry out a linearisation, and toemphasize or suppress specific parts of the scattered light curve, forexample, in order to evaluate separately scattered light or regularlyreflected light.

The advantages of the present invention are mainly found in thefollowing facts:

the surface quality is determined as an average over a measuring spot;

integral characteristic values S₁ or S₂ are formed which allow areliable and accurate statement even in the case of randomly formedscattered light curves, and the type of distribution, for exampleGaussian distribution, does not have to be known;

the characteristic value formation includes all detectors, and chancefluctuations in the scattered light curve, as caused by alignment errorsor statistical irregularities of the surface, are averaged out;

the integral characteristic values are invariant to a swing in thescattered light distribution in the measuring plane;

optical quality features of the surface, such as, for example, thescatter behaviour and structural features are exactly described by thecharacteristic values S₁ or S₂ ;

there is a close, very effectively reproducible connection between thecharacteristic values S₁ or S₂ and standardized roughness characteristicquantities when the fabrication conditions are known;

by adapting the solid angle detected by the measuring apparatus to theangle of the scattered radiation, a large measuring range is achievedwhich extends from R_(a) >0.01 μm to R_(a) <10 μm, oriented at theaverage roughness value R_(a) ;

separate determination of transverse and longitudinal roughness ispossible by rotating the measuring plane;

the measuring arrangement is insensitive to changes in spacing due tothe generally perpendicular irradiation;

the measurement may be carried out with gentle contact or free ofcontact manually or automatically even at difficulty accessiblemeasuring points; and

measurements on stationary and on moving surfaces are possible.

The characteristic values S₁ or S₂ may be determined in anelectronically analog or digital manner. It is preferred to calculatethe characteristic value in a computer because this easily allowsmeasured data to be stored, and allows an interactive operation controland a comprehensive documentation of the measuring results.

In order to adjust a specific characteristic line path of thecharacteristic values S₁ or S₂ over a range of the surface qulity, thecharacteristic values are to be multiplied in the computer with a scalefactor and/or powers of the characteristic values are to be used.Standardized roughness characteristic quantities such as the averageroughness R_(a) or the averaged roughness depth R_(z) are determinedindirectly via measuring procedures. Surfaces of known roughness aremeasured optically and the characteristic quantities S₁ or S₂ arerelated to stylus characteristic quantities. The relevant characteristicline is stored in a computer. In order to achieve a high measuringaccuracy, it is appropriate to measure and store several characteristiclines corresponding to the conventional processing methods.

Another possibility is to vary the wave length range of the measuredradiation in order to adjust different scatter angle ranges. Thescattering on surfaces of a specific roughness is greater with shortwave lengths, for example UV light, compared to long wave lengths, forexample IR light. The radiation used does not have to be monochromatic,but it may extend over a greater wave length range, for example, 100 nm.

Another possibility is to use polarized measured radiation. During thereflection of polarized radiation, rough surfaces exhibit a behaviourwhich differs from smooth surfaces and from which the surface qualitymay be deduced by considering the orientation of the polarisation planeof the radiation to the structure of the surface. Thus, thecharacteristic values S₁ or S₂ may be used as a measurement for thesurface quality from the polarisation condition of the radiation withinthe reflected radiation beam.

The invention is described in the following with reference toembodiments of the measuring apparatus. The drawings illustrateschematic views.

FIG. 1 illustrates a scattered light measuring apparatus according tothe present invention,

FIG. 2 illustrates a view of a measuring apparatus with a beam source ina measuring tube,

FIG. 3 illustrates a measuring apparatus with a flexible light guide,

FIG. 4 illustrates a measuring tube having a conical point,

FIG. 5 illustrates a measuring tube having a lateral beam outlet,

FIG. 6 illustrates a block diagram of the measuring system with acomputer for processing the measured values, and

FIG. 7 graphically illustrates the dependence of the characteristicvalue S₂ on the surface quality of different samples of surfaces.

In the measuring apparatus according to FIG. 1, a beam source 1 isprovided, for example, a diode emitting infrared light having a glasslens which is attached thereon and whose concentrated, generallyparallel radiation 9 passes through a beam splitter 2, a lens system 3and a plane-parallel glass plate 4. The measuring spot is restricted bya circular diaphragm 5 to about from 1 to 3 mm in diameter. The systemis completely encapsulated by the glass plate 4. The direction of theradiation 6 reflected by the surface 12 is changed by the refractivepower of a lens system 3 and the radiation 6 passes back through thebeam splitter which outwardly reflects as large a partial flux aspossible to the photoelectric detectors 8. Photodiodes or pyroelectricdetectors may be used individually or in the embodiments as a lineararray or a matrix array, as the detectors 8₁, 8_(i) to 8_(n). Thedetectors convert the radiation into electrical signals which arefurther processed in analog or digital manner, having been filtered andamplified, such that they state the scatter characteristic values S₁ orS₂ as a measurement for the surface quality. The path of the intensityof irradiation E is indicated on a screen 16, this path being measuredby the detectors 8₁ to 8_(n). In order to improve the disturbancesignal-to-noise ratio, the detectors are provided with an optical filterwhich narrows the spectral range of reception to the wave length rangeof the beam source.

A further improvement arises from working in variable light operation.For this purpose, the light of the beam source is pulsed electrically oroptically with a specific frequency and the measurement signal isevaluated in a frequency-selective manner. The measuring tube 7determines the solid angle with its dimensions, in particular its lengthand with the lens system 3 contained therein and with the glass plate 4,the reflected radiation being supplied to the detectors 8₁ to 8_(n) atthis solid angle. In the simplest case, the lens system 3 may beomitted. The solid angle then results from the geometrical data of themeasuring apparatus, substantially from the aperture angle of thedetector row 8. Lens systems having a positive focal distance enlargethe solid angle; dispersing lenses narrow this angle.

In the method according to the present invention, it is unnecessary toposition the detectors in the focal plane of the lens system or to carryout a real representation of the illuminated surface section, as aresult of which, advantages are provided in the dimensioning of themeasuring tube.

Several measuring tubes which may be mutually exchanged and which differin their optical characteristic data are provided for each measuringapparatus. This allows an adaption to the scatter angle range which isto be expected during a measurement and which results from the range ofthe surface quality of the surface to be examined. The range of thesurface quality is provided by the surface processing method which takesplace before measuring, for example, fine rotation, surface grinding orpolishing. The respective appropriate measuring tube is selected basedthereon.

According to FIG. 1, a cylinder lens 17 is inserted into the beam pathof the scattered light. This lens collects the spatially distributedscattered radiation into a band of light in the measuring plane which isdefined by the centre beams 9 and 10 and in which is located the row 8of detectors. As a result of this measure, the disturbing influence ofthe beam scatter on convex surfaces is reduced and the useful radiationflux is increased. This cylinder lens may be integrated into the lenssystem 3. It is even advantageous to construct the lens system 3 from asystem of crossed cylinder lenses, the focal distance of which isselected such that the scattered radiation is detected in one plane atthe angle which is most favourable for determining the surface qualityand in the other plane, the reflected radiation is collected into a bandof light in which the detectors 8 are located. In the case of lenseshaving a high refractive power, it is appropriate to flatten them in thecentre, so that the incident beam remains unchanged, while thedispersively reflected radiation is refracted by the curved lenssurfaces. This optical intervention is compensated during electronicprocessing of the measured values.

Another photoelectric detector 11 (reference detector) measures thepart, branched off by the beam splitter 2, of the beam 9 coming from thebeam source 1. The beam intensity fluctuations thereof are compensatedin the measured data processing operation via a quotient and/orsubtraction circuit. The measurement may take place with slight contactin that the small manageable scattered light measuring apparatus ispositioned with the measuring tube 7 generally perpendicularly on thesurface 12 to be measured. Since the supporting surface of the measuringtube is relatively large and only small forces are prevalent, thesurface to be measured is substantially prevented from being damaged.For measuring very sensitive surfaces, the supporting surface of themeasuring tube should be made of a non-rigid material, for example, aplastics material. Contact-less measurement is achieved by adjusting asmall measuring spacing 13 and thus, a measurement is also possible onmoving surfaces. This type of operation may also be advantageouslyapplied in automatic apparatus.

In order to determine the surface characteristic quantities, termedtransverse or longitudinal roughness in roughness measuring technologyon directional rough structures, for example, on grooved roughness, themeasuring device may be oriented with its measuring plane transverselyor longitudinally to the direction of the grooves. Another possibilityof measuring the direction-dependent scatter characteristics of asurface without rotating the measuring device about 90° during thismeasurement lies in a matrix arrangement consisting of several rows ofdetectors or in an arrangement of crossed rows of detectors. In thelatter case, a second row of detectors is positioned perpendicularly tothe row 8 of detectors and this second row detects the reflectedradiation perpendicularly to the measuring plane. A possibility ofexpanding the measuring range of the apparatus provided by the geometryand the optical system of the measuring tubes consists in changing thewave length of the radiation used. For this purpose, either the light ofthe beam source 1 is monochromatic, or a filter is used to screen aspecific wave length range from a beam source which has a broadspectrum.

Another possibility is to measure scattered light using polarizedradiation. For this purpose, a polarisation filter 14 is provided in thearrangement to produce polarized radiation and a polarisation filter 15is also provided to analyse the reflected radiation.

According to FIG. 2, the measuring apparatus according to FIG. 1 ismodified such that the beam source 1, for example a small semiconductorbeam source, is located in the measuring tube 7. The advantage of thisarrangement is the fact that the beam of rays from the beam source 1does not enter via the lens system 3, but the scattered radiation isdetected by the lens system. As a result of this, the scatteredradiation may be detected in a large solid angle which is necessary forthe coarse roughness range. In this arrangement, the beam splitter 2 ispositioned between the beam source 1 and the reference diode 11, whilethe row 8 of detectors is located in the housing of the apparatus.

Where there are unfavourable spatial conditions, it is advantageous touse a light guide, for example, in order to measure inside surfaces ofwork pieces or inside the processing machine using the apparatus. Anordered optical-fibre bundle may be used as a light guide, the crosssection of which may be rectangular or circular and/or fibre opticalcross-section transducers may be used. In the arrangement according toFIG. 1, the light guide is preferably inserted between the measuringtube 7 and the beam splitter 2 and it guides both the incident radiationas well as the reflected radiation. A light guide may also be used inthe arrangement according to FIG. 2 for guiding the reflected radiation.It is appropriately positioned between the measuring tube 7 and the row8 of detectors.

FIG. 3 illustrates an arrangement in which the beam source 1 is againlocated in the measuring tube 7 and is inserted directly into the lenssystem 3. Since the beam splitter has been omitted, the useful radiationof the beam source is increased. In this case, the reflected light fluxis supplied to the detectors 8 via a flexible light guide 18. In orderto illuminate the surface section to be measured, another light guidehaving a small diameter may be used which is positioned concentricallyto the light guide 18. This additional light guide is to be providedseparately from the light guide 18 in the housing of the apparatus andit allows the light of a high power beam source to be supplied to themeasuring point.

FIG. 4 illustrates an embodiment of a measuring tube having a conicalmeasuring point. This shape of the measuring tube is suitable forspatially confined measuring points.

FIG. 5 schematically illustrates another embodiment of the measuringtube, in which the light escapes at the side through a deflecting mirror19, which is advantageous, for example, for measurements in a bore or ina groove of a work piece.

FIG. 6 illustrates a block diagram of the measuring system. A computeris used to process the measured values. Several scattered light sensorsmay be connected to this basic electronic device. The measuringprocedure is clarified in the block diagram. A light-emitting diode isfed by a power source and it illuminates the surface to be measured ofthe measuring object. The photodiodes of a linear array convert thereflected light flow into electrical signals which are supplied to theelectronics (shown with interface) via multiplexers and controlled bythe computer. The signals are filtered in the electronics, amplified andare available to the computer for further processing as digital valuesafter an analog-digital conversion. The result of the measurements isrecorded or presented graphically on a screen or on a plotter.

Characteristic curves are shown in FIG. 7 which have been produced fromcomparative measurements of the method according to the presentinvention with stylus measurements on surface-ground and polishedsurface samples. The scatter characteristic value S₂ is dimensionlessand is plotted, multiplied with a scale factor against the averageroughness value R_(a) which was measured using a stylus device.

The apparatus which has been described is provided for determining thesurface quality of flat, concave or convex metallic surfaces. Inaddition thereto, it is possible using this measuring apparatus to alsodetermine the surface quality of parts made of other materials, forexample, semiconductor materials, plastics and porcelain.

A measuring method similar to the one according to the present inventionmay also be used for determining the surface quality or the scatterbehaviour of work pieces made of transparent material, for example,glass. In this method, the work piece to be examined is illuminated fromthe work piece surface opposite the measuring head and the penetratinglight is spread and scattered by the irregular structures. The scatteredlight distribution is then generally measured in the same manner asdescribed above for the reflected scattered light.

We claim:
 1. Apparatus for determining the roughness and structure,respectively, of a surface area of workpieces, said apparatus comprisinga source of light having a beam, the rays of which are substantiallyparallel and directed substantially perpendicularly onto the surfacearea to be examined; a plurality of detectors arranged in an array sothat each said detector senses the light scattered from said surfacearea within a given solid angle to measure the light intensity at ascattering angle in the distribution different from that measured by theother detectors, and an analyzing circuit means responsive to theoutputs of said detectors to determine an integral characteristic valuefrom the values of intensity measured by said detectors and from thescattering angles to thereby analyze and determine the surface roughnessand structure, respectively, in the following manner:(a) at firstdetermining standardized light intensity values p_(i) according to theequation ##EQU2## wherein D_(i) is the light intensity of the scatteredlight measured by the respective detector i, n is the number ofdetectors, and g_(i) is a correction factor for the respective lightintensity value D_(i) ; (b) then determining an average angle w of thescattered light by the detectors i from the values p_(i) and thescattering angles according to the equation: ##EQU3## wherein w_(i) isthe scattering angle at which the respective detector i measures theintensity of the scattered light; and (c) at last determining anintegral characteristic value S_(x) from from the value w and the valuesp_(i) and w_(i) according to the equation: ##EQU4## where x=1 or
 2. 2.The apparatus according to claim 1 including means for varying the wavelength of the measured radiation to change the scatter angle of thereflected radiation.
 3. The apparatus according to claim 1, includingmeans for polarizing the incident beam on the surface area to beexamined.
 4. The apparatus according to claim 3, including an analyzerin the path of said incident beam and a polarizer arranged in the pathof said reflected beam.
 5. The apparatus according to claim 1, includingmeans for rotating the measuring plane to establish adirectional-dependence of the reflection behaviour of surfaces having adirected (anisotropic) rough structure.
 6. The apparatus according toclaim 1, including means for supplying at least some of the incidentradiation to a reference photodetector and compensating for fluctuationsin the beam intensity by the output signal of the photodetector via ananalog or digital circuit.
 7. The apparatus according to claim 1,comprising a measuring tube positioned approximate to the surface to beexamined, said tube having a diaphragm and the solid angle of thescattered radiation which may be detected by the detectors is determinedby the length and/or optical characteristics of the tube.
 8. Theapparatus according to claim 7, including a lens system within saidmeasuring tube.
 9. The apparatus according to claim 8, wherein said lenssystem includes a cylinder lens for collecting the scattered radiationinto a band of light, and said photoelectric detectors are positioned ina row, as a linear photo array are located.
 10. The apparatus accordingto claim 9, wherein the lens system is formed from crossed cylinderlenses, one of which detects the scattered radiation at the respectivesolid angle and the other collects the scattered radiation into a bandof light in which the row of detectors is positioned.
 11. The apparatusaccording to claim 8 wherein the lenses of the lens system are flattenedin their centre, so that the incident beam of rays is unchanged whilepassing through the system, but the dispersively reflected radiation isrefracted by the lenses.
 12. The apparatus according to claim 7, whereinthe measuring tube is tightly sealed from said diaphragm by aplane-parallel glass plate.
 13. The apparatus according to claim 7,including a beam splitter positioned in the path of the incident beam tothe reference detector and for deflecting at least a part of thereflected radiation to the row of measuring detectors.
 14. The apparatusaccording to claim 7 wherein the source of light is very small, and ispositioned in the measuring tube between the lens system and thediaphragm.
 15. The apparatus according to claim 14, wherein the lightsource is integrated into the lens system.
 16. The apparatus accordingto claim 7, including a light guide provided between the measuring tubeand the row of detectors for guiding the incident beam and/or thereflected radiation.
 17. Apparatus according to claim 1 wherein thelight directed from said source of light to said surface area is passedthrough the same lens as said light scattered from said surface area anddirected onto said detectors.
 18. Method for determining the roughnessand structure, respectively, of a surface area of workpieces,particularly of metallic workpieces, wherein said method comprises:(a)directing a beam of light, the rays of which are substantially paralleland substantially perpendicular onto the surface area to be examined;(b) detecting the light scattered from said surface area at each onescattering angle of a plurality of scattering angles w_(i) by means ofan array of n detectors; (c) measuring the individual detector signalsD_(i) of each of the detectors to obtain a plurality of detectorsignals; (d) determining standardized detector signals p_(i) by summingup said individual detector signals D_(i) to obtain the sum of saidindividual detector signals and dividing each of said individualdetector signals D_(i) by said sum of said individual detector signalsto obtain a plurality of quotients p_(i) as standardized detectorsignals associated with each one of said detectors; (e) determining anaverage angle w of the scattered light from the standardized detectorsignals p_(i) and the scattering angles w_(i) ; according to the formula##EQU5## (f) determining a characteristic value S₁ of roughness andstructure, respectively from the average angle w and the values p_(i)and w_(i) by subtracting said average angle w from each of saidindividual scattering angles w_(i) to obtain a plurality of differencesw_(i) -w of said scattering angles and said average angle w, multiplyingthe absolute values |w_(i) -w| of each of said differences by therespective of said standardized detector signals p_(i) to obtainproducts |w_(i) -w|.pi, and summing up the plurality of said products|w_(i) -w|.pi to obtain said characteristic value S₁ of roughness andstructure, respectively.
 19. Method for determining the roughness andstructure, respectively, of a surface area of workpieces, particularlyof metallic workpieces, wherein said method comprises:(a) directing abeam of light, the rays of which are substantially parallel andsubstantially perpendicular onto the surface area to be examined; (b)detecting the light scattered from said surface area at each onescattering angle of a plurality of scattering angles w_(i) by means ofan array of n detectors; (c) measuring the individual detector signalsD_(i) of each of the detectors to obtain a plurality of detectorsignals; (d) determining standardized detector signals p_(i) by summingup said individual detector signals D_(i) to obtain the sum of saidindividual detector signals and dividing each of said individualdetector signals D_(i) by said sum of said individual detector signalsto obtain a plurality of quotients p_(i) as standardized detectorsignals associated with each one of said detectors; (e) determining anaverage angle w of the scattered light from the standardized detectorsignals p_(i) and the scattering angles w_(i) ; according to the formula##EQU6## (f) determining a characteristic value S₂ of roughness andstructure, respectively from the average angle w and the values p_(i)and w_(i) by subtracting said average angle w from each of saidindividual scattering angles w_(i) to obtain a plurality of differencesw_(i) -w of said scattering angles w_(i) and said average angle w,determining the second powers of the absolute values |w_(i) -w| of eachof said differences, multiplying second powers of the absolute values|w_(i) -w| of each of said differences by the respective of saidstandardized detector signals p_(i) to obtain products |w_(i)-w|².p_(i), and summing up the plurality of said products |w_(i) -w|².p_(i) to obtain said characteristic value S₂ of roughness andstructure, respectively.
 20. Method for determining the roughness andstructure, respectively, of a surface area of workpieces, particularlyof metallic workpieces, wherein said method comprises:(a) directing abeam of light, the rays of which are substantially parallel andsubstantially perpendicular onto the surface area to be examined; (b)detecting the light scattered from said surface area at each onescattering angle of a plurality of scattering angles w_(i) by means ofan array of n detectors; (c) measuring the individual detector signalsD_(i) of each of the detectors to obtain a plurality of detectorsignals; (d) determining corrected standardized detector signals p_(i)by multiplying each one of said detector signals D_(i) by each one ofindividual correction factors gi associated to each one of saiddetectors, summing up the obtained products D_(i).gi and dividing eachof said products D_(i).gi by the sum of said products D_(i).gi, toobtain said corrected standardized detector signals P_(i) ; (e)determining an average angle w of the scattered light from the correctedstandardized detector signals p_(i) and the scattering angles w_(i) ;according to the formula ##EQU7## (f) determining a characteristic valueS₁ of roughness and structure, respectively from the average angle w andthe values p_(i) and w_(i) by subtracting said average angle w from eachof said individual scattering angles w_(i) to obtain a plurality ofdifferences w_(i) -w of said scattering angles w_(i) and said averageangle w, multiplying the absolute values |w_(i) -w| of each of saiddifferences by the respective of said corrected standardized detectorsignals p_(i) to obtain products |w_(i) -w|.pi, and summing up theplurality of said products |w_(i) -w|.pi to obtain said characteristicvalue S₁ of roughness and structure, respectively.
 21. Method fordetermining the roughness and structure, respectively, of a surface areaof workpieces, particularly of metallic workpieces, wherein said methodcomprises:(a) directing a beam of light, the rays of which aresubstantially parallel and substantially perpendicular onto the surfacearea to be examined (b) detecting the light scattered from said surfacearea at each one scattering angle of a plurality of scattering anglesw_(i) by means of an array of n detectors; (c) measuring the individualdetector signals D_(i) of each of the detectors to obtain a plurality ofdetector signals; (d) determining corrected standardized detectorsignals p_(i) by multiplying each one of said detector signals D_(i) byeach one of individual correction factors g_(i) associated to each oneof said detectors, summing up the obtained products D_(i).g_(i), anddividing each of said products D_(i).g_(i) by the sum of said productsD_(i). g_(i), to obtain said corrected standardized detector signalsp_(i) ; (e) determining an average angle w of the scattered light fromthe corrected standardized detector signals p_(i) and the scatteringangles w_(i) ; according to the formula: ##EQU8## (f) determining acharacteristic value S₂ of roughness and structure, respectively fromthe average angle w and the values p_(i) and w_(i) by subtracting saidaverage single w from each of said individual scattering angles w_(i) toobtain a plurality of differences w_(i) -w of said scattering anglesw_(i) and said average angle w, determining the second powers of theabsolute values |w_(i) -w| of each of said differences, multiplyingsecond powers of the absolute values |w₁ -w| of each of said differencesby the respective of said corrected standardized detector signals p_(i)to obtain products |w_(i) -w|².p_(i), and summing up the plurality ofsaid products |w_(i) -wS|².p_(i) to obtain said characteristic value S₂of roughness and structure, respectively.